Capacitive, semiconductive sensors or transducers, used, for example, for sensing pressure or acceleration or other physical phenomenon, are known.
For example, capacitive pressure sensors are well known and are employed in capacitance transducers, microphones, rupture discs, resonators, vibrators and like devices. Many of the applications for such capacitive pressure sensors require that the sensors be extremely small, for example, of the order of eight millimeters by eight millimeters (8 mm.times.8 mm).
Silicon capacitive pressure transducers are known in the art. For example, U.S. Pat. No. 3,634,727 to Polye discloses one type in which a pair of centrally apertured, conductive silicon plates are joined together with a eutectic metal bond, such that the silicon disc plates flex with applied pressure, changing the capacitance of the aperture interstice and providing a capacitive-type signal manifestation of pressure magnitude. This form of pressure transducer thus relies on the pressure-induced deflection of a thin diaphragm, in which the diaphragm deflection as a function of fluid pressure causes a variation in the distance between a pair of surfaces which effectively form the plates of a variable capacitor. Other examples of such silicon pressure sensors or transducers are included in the U.S. patents listed below.
Thus, capacitive pressure sensors are well known and are employed in capacitance transducers, microphones, rupture discs, resonators, vibrators and like devices. Some of the capacitive pressure sensors require that the sensors be extremely small, for example, of the order of about eight millimeters by eight millimeters (8 mm.times.8 mm) or less and are typically made in a silicon-glass-silicon sandwich design.
An exemplary prior art, silicon-glass-silicon pressure sensor design of the sandwich type is illustrated in FIG. 1A. Such a sensor or transducer 10, which typically is generally square in its exterior configuration but often at least generally and preferably circular or cylindrical in shape for its inner, operative substructure, generally as can be seen in FIG. 1A, includes an upper, conductive, square, flexible, appropriately doped, silicon diaphragm 11 and a lower or bottom, conductive, appropriately doped, silicon base or substrate 12 with a non-conductive dielectric layer and spacer 13 (made of, for example, borosilicate glass) between them.
A closed, evacuated, hermetically sealed, reference cavity, chamber or interstice 14 is formed between the two silicon layers 11, 12. The chamber 14 is typically at a zero pressure or vacuum, or can be sealed at a higher reference pressure, at which reference level the diaphragm 11 is parallel to the silicon substrate 12, with typically a two micrometer spacing between the two.
A centrally located, typically circular pedestal or mesa 12A extends into the typically generally cylindrical, closed chamber 14 with a thin, insulating layer of glass covering the top of the mesa. The circular mesa 12A serves as a counter-electrode to the deformable capacitor plate or diaphragm 11.
The mesa 12A extends up from the main surface of the silicon substrate 12 an exemplary six and a half micro-meters, while having an exemplary diameter of one hundred and fifty thousandths (0.150") of an inch.
For further general background information on the exemplary application for the present invention, namely, in the design of the peripheral areas of the diaphragm 11 of the pressure sensor 10, it is noted that the wall(s) 16 might typically have a horizontal, lateral or radial thickness of, for example, thirty-six thousandths (0.036") of an inch with a height of, for example, nine (9) micrometers, while the separately applied, insulating, mesa layer of glass is only about a half a micrometer thick.
The silicon diaphragm 11 and the silicon base 12 may typically be square [with corners removed for the purpose of providing access for electrical contacts to the layer(s), as illustrated], having a horizontal length of an exemplary two hundred an sixty thousandths (0.260") of an inch on an edge, while the spacer wall 16 can have an inner diameter of an exemplary one hundred and ninety thousandths (0.190") of an inch. The outer, side surface of the wall spacer 16 can either follow the basic square configuration of the silicon layers or have an outer circular configuration.
It should be understood that the simplified FIG. 1A hereof for practical purposes of illustration is not at all to relative scale, as the glass wall or spacer 13/16 is only typically nine micrometers high, in contrast to the thicknesses of the silicon layers 11 & 12, which typically are eight thousandths (0.008") of an inch and fifty thousandths (0.050") inches thick, respectively, for an exemplary fifty (50 psi) pounds per square inch pressure measuring unit.
Additionally, for still further general background purposes, it is noted that an exemplary, prior art, three plate, silicon-glass-silicon (SGS) device is particularly described in assignee's U.S. Pat. No. 4,467,394 of Grantham & Swindal, the inventors hereof, issued Aug. 21, 1984.
Some exemplary, prior art, U.S. Pat. Nos. in the field of capacitive pressure sensors or transducers, including the '394 patent, all owned by the assignee hereof, are listed below:
______________________________________ Issue U.S. Pat. No. Title Inventors Date ______________________________________ 4,530,029 Capacitive Pres- C. D. Beristain 07/16/85 sure Sensor With Low Parasitic Capacitance 4,517,622 Capacitive Pres- B. Male 05/14/85 sure Transducer Signal Condition- ing Circuit 4,513,348 Low Parasitic D. H. Grantham 04/23/85 Capacitance Pres- sure Transducer and Etch Stop Method 4,467,394 Three Plate D. H. Grantham 08/21/84 Silicon-Glass- J. L. Swindal Silicon Capactive Pres- sure Transducer 4,463,336 Ultra-Thin Micro- J. F. Black 07/31/84 electronic Pres- T. W. Grudkowski sure Sensors A. J. DeMaria 4,415,948 Electrostatic D. H. Grantham 11/15/83 Bonded, Silicon J. L. Swindal Capacitive Pres- sure Transducer 4,405,970 Silicon-Glass- J. L. Swindal 09/20/83 Silicon Capacitive D. H. Grantham Pressure Trans- ducer ______________________________________
In state of the art pressure sensors for aerospace applications, a linear frequency output from the unit is useful for interfacing with data management computers and controls units. However, such a linear response is not obtained with simple diaphragm structures, such as those of the prior art. Consequently, calibrations for the sensor must be stored in computer memory.
In the relatively simple diaphragm structures of the prior art used for capacitive pressure sensing, the capacitance changes in a non-linear fashion with change in pressure, because the central region 17 of the flexed diaphragm 11 being moved under external pressure assumes a curved shape (note FIG. 1) and moves in a non-planar manner. The result is that the frequency output desired (which is inversely proportional to the capacitance; f=1/C) is not linear in pressure.
To overcome this problem of sensor non-linearity, the present invention teaches a method and structural approach for linearizing the frequency output as a function of pressure.
Although it has been suggested to provide a "hinge" in association with a diaphragm in silicon pressure sensors, noting, for example, assignee's U.S. Pat. No. 4,513,348 of Grantham, such used solely a glass dielectric as the hinging medium, which is not as satisfactory as the use of silicon as at least the primary, if not sole, medium, as in the present invention.